1. Field of the Invention
The present invention relates to an EL (electroluminescent) device which emits light when applied with an electric voltage.
2. Description of the Prior Art
An EL device which includes therein a lightemitting layer, or an electroluminescent layer, sandwiched between electrodes to emit light when D.C. or A.C. voltage is applied between the electrodes is disclosed, for example, in U.S. Pat. No. 4,140,937.
To make this device, a transparent electrode 2 is formed from transparent conductive material, such as tin oxide and indium oxide, on a glass substrate 1 as shown in FIG. 5 by evaporation or sputtering. Then, a substance made into paste form by dispersing such a phosphor material, which is composed of zinc sulfide or the like as the base material and copper to become luminescence centers and the like as active impurities added thereto, in such an organic binder as cellulosic resin is applied to aforesaid electrode and dried to be formed into a light-emitting layer 3. Over this layer is formed a back electrode 4 made of conductive metal with a good light reflecting property such as aluminum. When D.C. or A.C. voltage is applied between the transparent electrode 2 and the back electrode 4, a high electric field is developed within the light-emitting layer 3 and electrons in the conductor are excited and accelerated by the high electric field so as to be sufficiently energized to excite the aforesaid active substance, i.e., copper luminescence centers, and thus light is emitted when the excited copper luminescence centers return to the ground state.
Although such an EL device has an advantage in that its power consumption is lower than other surface luminescent devices such as a plasma display panel and fluorescent display tube, it has a disadvantage that the EL device as a whole becomes heavier in weight since the glass substrate 1 is heavy. Therefore, although such EL elements were suitable for the use in a fixed state, they were not suitable as light sources for displays, for example, for a show window, Christmas tree, or the like, which are used in a suspended state.
On the other hand, an EL device employing a light-transmitting sheet member instead of such a glass plate 1 is shown, for example, in U.S. Pat. No. 3,509,401.
To provide this device, as shown in FIG. 6, a light-transmitting sheet member such as a flat polyester film is used as its substrate, and by applying thereto transparent conductive material, such as tin oxide and indium oxide, by means of evaporation, sputtering, or the like, and cutting the film into a desired luminescent shape while providing the same with a transparent electrode terminal 10a, a transparent electrode 10 is thus formed.
Then, a moistureproofing member 12 made of a thermoplastic high polymer light-transmitting sheet member, such as chlorotrifluoroethylene film or composite film of chlorotrifluoroethylene film and polyethylene film, which is made to be somewhat larger than the transparent electrode 10 and provided with bonding agent 11 of olefinic series or the like on one side thereof, is disposed such that the surface applied with the bonding agent 11 is in contact with the mentioned light-transmitting sheet member forming the transparent electrode 10.
Then, a light-emitting layer 13 is formed on the transparent electrode 10 by means of screen printing or the like.
Thereafter, on the light-emitting layer 13 is disposed a back electrode 14 which is of the same constitution as the transparent electrode 10 or made of conductive metallic material having a good light reflecting property as aluminum. At this time, an electrode terminal 14a which is formed to be integral with the back electrode 14 and led out is disposed so as not to overlap the electrode terminal 10a of the transparent electrode 10.
Then, a moistureproofing member 16 of the same constitution as the aforementioned moistureproofing member 12 with bonding agent 15 of olefinic series or the like provided on one side thereof is disposed on the back electrode 14 such that the side applied thereon with the bonding agent 15 comes in contact with the back electrode 14 and both the electrode terminals 10a, 14a for the electrodes 10, 14 are partly exposed.
Finally, two moistureproofing members 12, 16 are fused together by a laminating method or the like under the heating condition at higher temperature than the plasticizing temperature of the same, and thus, the EL device is completed. The EL device of such constitution applied with the aforementioned voltage between its electrode terminals 10a, 14a emits light similarly to the previous one.
Since the above mentioned EL device uses, as the transparent electrode 10, a light-transmitting sheet member with a transparent conductive material disposed thereon, instead of the glass substrate 1 (refer to FIG. 5), it has an advantage that the EL device is much lighter in weight.
However, the light-transmitting sheet member used as the substrate for the transparent electrode 10 and the moistureproofing member 12 are inferior to the glass substrate 1 in the light-transmitting property, and further, the bonding agent 11 interposed between the transparent electrode 10 and the moistureproofing member 12 disturbs the transmission of light, and therefore, quantity of the light outwardly emitted from the light emitting layer 13 of the EL device is greatly reduced. Such low efficiency in the light emission was a disadvantage of this type of EL device